Tumour size determines both recurrence-free survival and disease-specific survival after surgical treatment for thymoma

Objectives: The tumour, node and the metastasis (TNM) staging system for thymic epithelial tumours was adopted by the Union for International Cancer Control (UICC) in 2016. Although the T factor is defined by the invasive nature of a thymoma, tumour size is not considered. The aim of this study was to examine the clinical importance of tumour size using a nationwide retrospective database of cases treated from 1991 to 2010 compiled by the Japanese Association for Research of the Thymus. Methods: Tumour size was evaluated by the maximum diameter shown by computed tomography imaging prior to resection. Tumour size was available for 2083 thymoma patients undergoing upfront surgical treatment. The tumour size ranged from 0.6 to 19.4 cm (mean 5.1 cm, median 4.9 cm). Harrell's C-index was adopted to determine the cut-off value of the tumour size in 0.5-cm increments. Results: The highest C-index value (0.7760) was obtained in terms of recurrence-free survival after the complete resection when the cut-off value was set at 5.0 cm. The 10-year recurrence-free survival rate was 93.8% in patients with a tumour ≤5.0 cm and 84.3% in patients with a tumour >5.0 cm (P < 0.0001). The highest C-index value (0.8885) in terms of disease-specific survival was obtained when the cut-off value was set at 8.0 cm. The 10-year disease-specific survival rate was 98.8% in patients with a tumour <8.0 cm and 90.1% in those with a tumour ≥8.0 cm (P < 0.0001). The Cox's proportional hazard model analysis showed that the tumour size and the TNM-based pathological stage were independent factors to determine both recurrence-free survival and disease-specific survival. Conclusions: Tumour size is an important prognostic factor and should be considered when determining the treatment strategy for thymoma patients.